A runner for a Francis turbine or a pump-turbine is generally designed so that it operates smoothly and efficiently for an optimum operating condition defined by predetermined values of head, flow, speed and runner diameter. At optimum or near-optimum operating conditions of a Francis turbine or a pump-turbine operated as a turbine, the highest efficiencies will be obtained, and the water will leave the runner without rotation or with only slight rotation. At operating conditions deviating significantly from the optimum condition, the efficiencies will be lower, and certain undesirable disturbances, e.g., pressure pulsations, output fluctuations and an increased sound level, will occur. Thus, it is undesirable, and in some cases may be impossible, to operate the turbine at operating conditions deviating substantially from the optimum conditions because the disturbances can become severe enough seriously to damage the turbine or equipment associated with it.
An important cause of the disturbances that occur under conditions deviating from the design conditions is the fact that the water leaves the runner with rotation, i.e., a circumferential component of flow or a vortex flow. At optimum operating conditions, the rotation of the water is prevented by designing the runner such that the peripheral velocity at any point on the trailing edge of a runner vane combines with the velocity of flow relative to the rotating runner in such a manner that the resulting absolute velocity of the water is directed substantially axially. However, when the operating conditions deviate from the optimum condition, the velocity of the water relative to the vanes is altered, thus resulting in an absolute velocity of the water having a circumferential component.
More specifically, when the turbine load exceeds the optimum condition, and is accommodated such as by increasing the head on the runner or opening the wicket gates to increase flow, the velocity of the water relative to the vanes is increased, thereby resulting in an absolute water velocity having a component in the direction opposite to that of the peripheral velocity of rotation of the runner and forming a vortex which rotates in a direction opposite to that of the runner. When the turbine load is being optimum or "design", the velocity of the water relative to the vanes is reduced, thereby resulting in an absolute velocity having a component in the direction of the peripheral velocity of rotation of the runner and forming a vortex (a circumferential flow) which rotates in the direction of rotation of the runner. If the rotation is severe, the pressure at the center of the rotating water mass, i.e., the vortex, will be so low that cavitation occurs. Moreover, the vortex is not coaxial to the axis of rotation of the runner but makes a precessional movement about it.
To obtain a range of operating conditions as large as possible with a minimum of disturbance, most of the known solutions have concentrated on the design of the runners, especially the shape of the runner vanes. British Patent No. 799,013 describes another approach, namely to decrease the intensity of the vortex by providing fins positioned on the portion of a crown tip furthest from the runner vanes. Because of their relatively small radial diameter and their interconnection at the center of the shaft of the crown tip, the fins described and illustrated in the British patent do not significantly improve the flow stability and efficiency of the runner.